1. Field of the Invention
This invention relates to sodium percarbonate compositions having a long shelf life or an excellent stability property.
2. Description of the Prior Art
Sodium percarbonate is a known substance useful as a bleaching agent or oxidizing agent. Sodium percarbonate and sodium perborate are typical oxygen-type bleaching agents for domestic and commercial clothes washing purposes. Sodium percarbonate has the formula: 2Na.sub.2 CO.sub.3 .3H.sub.2 0.sub.2 . It is generally prepared by reacting sodium carbonate with hydrogen peroxide.
Although the bleaching power of sodium percarbonate is a little lower than that of chlorine-type bleaching agents at ambient temperature (20.degree.-25.degree.C), sodium percarbonate has the advantage that it can be applied to synthetic fibers, animal fibers, resin-treated fibers and fluorescent brightening agent-treated fibers without causing yellow staining or otherwise damaging the fibers. A sufficient bleaching effect thereof can be obtained by raising the treatment temperature or by using a decomposition promoting agent in combination therewith. Thus, sodium percarbonate is used particularly as a bleaching agent for domestic and commercial clothes washing purposes.
The reasons why sodium percarbonate is attracting attention as an ingredient in general purpose clothes washing detergents and domestic bleaching agents are that its decomposition products cause no environmental pollution and it can be put to practical use in a variety of different ways without difficulty.
However, in comparison with sodium perborate, sodium percarbonate has, disadvantageously, a lower stability during storage and it loses available oxygen rapidly during storage. Due to the high compatibility of sodium percarbonate with water, the surface thereof becomes wetted or absorbs water even in an atmosphere of very low humidity whereby it is decomposed. In the presence of metal ion such as iron, copper, manganese or cobalt, the decomposition of sodium percarbonate is further promoted. Thus, sodium percarbonate is inferior to sodium perborate with respect to stability.
When sodium percarbonate is stored by itself in a closed vessel, it exhibits a shelf life equivalent to that of sodium perborate. However, when sodium percarbonate is stored in the form of a mixture with a detergent, or in an open vessel, the storage stability thereof is reduced due to its high hygroscopicity, even through it has a high solubility.
Another disadvantage of sodium percarbonate is that it forms lumps or cakes if it is allowed to stand in the form of a powder. This lumping poses a practical problem when it is intended to use percarbonate in the form of a powder.
In view of the circumstances described above, it is desired to provide sodium percarbonate which can be stored in an open vessel without fear of decomposition or lumping and which is free from any practical problem of insufficient shelf life, even when it is mixed with other ingredients of detergent or bleaching compositions.
Previously proposed processes for stabilizing sodium percarbonate include coating sodium percarbonate with a paraffin or polyethylene glycol having a molecular weight of 3,000 to 8,000. The former process wherein the paraffin is used is impractical, because the water-solubility of the sodium percarbonate is reduced remarkably. The latter process wherein polyethylene glycol is used does not provide long time stabilization of sodium percarbonate, because polyethylene glycol per se has a considerable hygroscopicity, even though the water-solubility of sodium percarbonate is maintained.
Another process which has been proposed comprises adding at least two stabilizers selected from the group consisting of phosphoric acids, silicic acids, ethylenediamine tetraacetate and nitrilo triacetate to the aqueous hydrogen peroxide solution used in the preparation of the sodium percarbonate. However, those stabilizers have no practical stabilizing effect on sodium percarbonate in the presence of water or a detergent, although they apparently improve the heat stability.
Still another process comprises coating sodium percarbonate with sodium pyrophosphate. This process is also insufficient with respect to its stabilizing effect on sodium percarbonate in the presence of water or a detergent, although the decomposition caused by heat is reduced.